Microwave heating has demonstrated itself to be a powerful technique for sintering various ceramics, especially through the past decade. Microwave heating may decrease the sintering temperatures and times dramatically, and is economically advantageous due to considerable energy savings. However, one of the major limitations is the volume and/or size of the ceramic products that can be microwave sintered because an inhomogeneous microwave energy distribution inside the applicator which often results in a non-uniform heating. Considerable research has gone into making microwave sintering technology commercially viable.
This disclosure sets forth three different types of products which can be handled by microwave heating to obtain sintering. The three different types of products refers to the form of the products, not the chemical makeup of the products. Indeed, the products can be made of the same constituent ingredients. They differ however primarily in the shape and hence the cohesive nature of the respective products.
The three product formats or forms include loose particulate material, i.e., a powder of a specified size, a molded product or a precast molded product. The distinction in the latter is that it is precast sufficiently that it requires no mold during sintering. It can be precast with a sacrificial wax or other adhesive which glues the particles together into a precast form. During sintering, the form is not changed in terms of shape, but the form is sustained although this is accomplished free or devoid of a confining mold. The molded product is a product which is held in a mold during sintering. One of the advantageous aspects of the molded products is that initial mold shaping of the particles making up the product can be accomplished at very low temperatures and pressures, i.e., substantially at room temperature and atmospheric pressure. Typically, a set of particles are joined in a mold again by a sacrificial wax or other material or alternately by the confines of the cavity mold itself. In either instance, the finished product is a structure which is sintered and yet which has a defined shape or profile. Examples abound as will be set forth below.
In all instances, it will be described so that the sintering process begins or acts on what are known as green materials. The term green materials refers to those which have been provided but have not been sintered. For particulate matter, they typically have the form of powders. Both in the molded and precast forms, one of the beginning materials is the requisite quantity of particles prior to molding, i.e., shaping into a desired form either by precast molding or sintering in a mold.
The preparation of loose material which is sintered defines small particles which can be used later in abrasive wheels and the like. Normally, these materials must be sintered to a specified grain size. In many applications, the quality or performance of the material is directly impacted by the grain size accomplished in the sintering process. In one aspect, grain size has an undesirable impact on the finished product. More specifically, this arises from the fact that additives often are placed in control quantities in the material prior to sintering so that the grain boundaries are defined by the additives. While there are additives available which do control grain size, the additives weaken or reduce the hardness of the finished product. Therefore such additives, while desirable in one aspect, are not desirable in other regards. The amount, nature, and dispersal of such grain boundary additives is a material factor, thereby providing a balanced mix of properties where the properties themselves result in some kind of compromise in the design of such sintered products. Effectively, grain boundary size is controlled only at a cost in sintered particle hardness.
Continuous microwave sintering of alumina is newly developed. One aspect of the continuous microwave sintering furnace is shown in FIG. 1. The microwave applicator is designed to focus the microwave field in the central area as uniformly as possible. A long cylindrical ceramic hollow tube contains the unsintered (or green) material which is fed into the microwave applicator at a constant feed speed. As the green material enters the microwave cavity, it is heated and gradually sintered while passing through the microwave zone. The heating rate, sintering time and cooling rate are controlled by the input microwave power, the feeding speed, and the thermal insulation surrounding the heated material. The ceramic hollow tube is also rotated during processing for uniform and homogeneous heating. As the green material passes through the high temperature zone, the particles are sintered entirely. Since the ceramic hollow tube is moved continuously in the axial direction during the processing, there is virtually no limitation to the length or volume of the product that can be processed by this technique. Consequently, it is, possible to scale up the volume of the ceramic products to be microwave sintered by this technique by implementing a continuous process.
This disclosure proves the continuous microwave sintering technique for small or large quantities of green material to make a desired shape or volume of material. The results show better physical properties than the conventionally processed material. The disclosure sets out three different product configurations. One form is a loose, unconsolidated particulate product, a second comprises a cold press shaped or configured particulate body shaped by a mold at minimal pressure, and a third form is a cold pressed, unconfined form of sufficient strength to hold its own shape. The three products are generally referred to below as sintered particles, molded products and precast products.
This disclosure is directed to a novel synthesis method for the manufacture of finished ceramics and/or ceramic/metallic composites utilizing the newly developed microwave processing. The process offers a faster, energy efficient route to manufacture extra hard products. Sintered particles prepared by this method exhibited greater micro Vickers hardness, even as much as 1500 kg/mm.sup.2, better crystalline uniformity and average grain size less than sintered materials processed in the conventional manner. One aspect of this invention relates to improved preparation of parts made of nitrides, carbides, and similar hard materials.
The present disclosure sets forth a sintering apparatus which can be used for sintered particles or for cast items (molded or precast). Examples will be given of all three. A molded part can be sintered by placing green particulate material in a mold or cavity. The mold is first filled with the green constituent materials. Hard wear parts can be made. As an example, tungsten carbide or silicon nitride particles are packed into a mold or cavity. An interspersed particulate binder metal, typically a cobalt alloy, is added in the mold or cavity. In the past, extreme heat with deleterious consequences was applied in the ordinary manufacturing process along with extremely high pressure to form a molded part. The resultant part is a matrix of hard particles which are held together by the melted alloy. The alloy serves as a binder which holds the shape of the finished part. By applying an adequately high pressure to the cavity and by also applying an adequately high temperature for a desired interval, molded parts were made in this fashion. Such wear parts have extremely long life. Examples of such wear parts include teeth (sometimes known as inserts) used in drill bits, nozzles for directing a flow or stream of fluid, deflector plates, scuff plates and the like. This process completely avoids such manufacturing equipment, thereby reducing cost and improving speed of fabrication.
The finished products are formed in a conventional manner using extreme heat and pressure. By contrast, such molded products can be made using the microwave sintering apparatus and method set forth in the present disclosure. The particulate materials are tamped into a cavity at a desired packing density without requiring any extremely high pressures. The cavity is formed in a tube of material which is transparent to microwave radiation. This transparent tube is then positioned in the microwave cavity of the sintering apparatus. Sintering occurs at a more rapid temperature increase yet is consummated at a lower temperature level. Moreover, the grain size within the solid part does not grow as great as normally occurs in a conventional liquid sintering process. Improved hardness and chip resistance is obtained with a smaller grain structure in the molded part. The alloy sinters the entire particulate mass in the mold to thereby furnish a wear part. Examples of this will be given below.
The particulate or green material is shaped at room or ambient temperature in a mold, a preliminary process called "cold pressing". The tamped or pressed particles are shaped by a low cost cavity or mold. If the particles are sufficiently adhesive, the precast particles (without mold). can be sintered; if crumbling occurs, the low cast mold can be exposed to the microwave field to sinter the mold contents.
By the use of the process of the present invention, it is possible to prepare a new variety of extra hard, shaped parts at considerably lower temperature with smaller grain size, higher hardness and density. The process of the present invention also uses microwave sintering to obtain higher heating rates to form better conventional products. In the process of the invention, microwave heat is generated internally within the material instead of originating from external heating sources and is a function of the material being processed. As temperature increases above a point, the dielectric loss begins to increase rapidly and the sintered part begins to absorb microwaves more efficiently. This also raises the temperature. Hence, heating rates are as high as 300.degree. C./minute. Both batch and continuous processing systems can be employed.
As a rule of thumb, the performance of the particulates with the same hardness, toughness and density improves with decrease in grain size. It is possible to achieve very small grain sizes with high hardness, toughness and density, using the microwave processes thereby improving the characteristics when compared to the conventional process. This process requires much lower temperature (less than about 1350.degree. C.) than conventional sintering techniques (around 1500.degree. C.).